Purification of refrigerant



Nov. 18, 1969 P. J. BQYKIN 3,478,529

PURIFICATION OF REFRIGERANT Filed April 17, 1968 g r 1 r N I N N* *l f" l r i -2 In cof :cr A

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P. J. BoYKlN BY 7W? 4% .ATTORNEYS United States Patent O 3,478,529 PURIFICATION OF REFRIGERANT Philip J. Boykin, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Apr. 17, 1968, Ser. No. 721,962 Int. Cl. FZSb 43/00 U.S. Cl. 62-85 7 Claims ABSTRACT OF THE DISCLOSURE A process for continuously or intermittently purifying an impure refrigerant in a cooling system thereby increasing the efliciency of refrigeration and reducing the cost of operation and equipment. The refrigerant is puried by vaporizing the lighter impurities in one zone and by removing the heavier impurities from the evaporator. The refrigerant associated with the heavier impurities is vaporized by using a portion of the hot compressed gases from the compressor as the heat source.

This invention relates to a process for continuously or intermittently removing impurities from a refrigerant in a refrigeration system thereby increasing the eflciency of refrigeration in order to reduce the cost of operation. In a further aspect, it relates to a process for decreasing the concentration of impurities associated with a refrigerant in a refrigeration system which utilizes a minimum amount of equipment.

It is a common practice to use lower alkanes as a refrigerant in a refrigeration system for industrial plants. Substantially pure alkane is generally employed for such uses because of the adverse effects of impurities on the efficiency of the refrigeration system. That is, as impurity concentration increases, the power required to compress and condense the vaporous refrigerant also increases, thereby increasing cost. It is therefore essential to provide a method for removing the impurities from the refrigerant to reduce the cost of operation. However, the equipment needed to accomplish the purification of the refrigerant has heretobefore been expensive to procure, maintain and operate.

Throughout the specification and claims the term lighter impurities refers to compounds which are associated with the refrigerant which have a lower boiling point than the refrigerant. Accordingly, the term heavier impurities is defined as compounds associated with the refrigerant which have a higher boiling point than the refrigerant.

Various methods have been developed to accomplish the purpose of removing the lighter Vand heavier compounds from the refrigerant. However, these methods usually result in a loss f valuable pure refrigerant along with reducing the concentration of impurities. Heretobefore, the heavier impurities have been purged from the system by utilizing a fractionation tower supplied with an external heat source to vaporize the refrigerant entrained within the heavier impurities. In addition, lighter impurities have been removed from the system by selectively condensing vaporous refrigerant from a mixture of the vaporous refrigerant and vaporous lighter impurities, and allowing the lighter hydrocarbon impurities to leave the system as vapor. However, the removal of the heavier impurities has required equipment which -has resulted in higher initial cost and higher operating cost for the system due to the heat requirement necessary to remove the heavier impurities.

I have discovered a process whereby refrigerant in a refrigeration system may be continuously or intermittently pured of higher and lower boiling point impurities in such a way as to optimize the efficiency of the system in "ice much less equipment than has hereinbefore been required, and therefore reduce the cost of refrigeration. Briefly, the invention consists of removing the lighter impurities by placing a mixture of the vaporous lighter impurities and vaporous refrigerant in indirect heat exchange with cold liquid refrigerant. The heat exchange effects a condensation of the vaporous refrigerant and vaporization of the cold liquid refrigerant, but allows the vaporous impurities to exit the" system. The heavier impurities are removed from the refrigerant by removing liquid refrigerant from a primary evaporator which is used to cool a process fluid. The liquid in the evaporator contains a high concentration of the heavier impurities. This liquid is placed in heat exchange relation with hot compressed refrigerant in order to vaporize the refrigerant from the heavier impurities. The liquid heavy impurities are then removed from the system.

Accordingly, it is an object of this invention to provide an improved process for the purication of refrigerant in a refrigeration system. It is another object of this invention to provide to the art a new method of removing impurities from a refrigerant which reduces the quantity of the equipment heretobefore required.

Other objects and advantages of the invention will be apparent from this disclosure, the appended claims, and the drawing.

The drawing is a schematic flow diagram which illustrates the process of the invention.

In accordance with the invention, a liquid refrigerant is passed to the shell section of an indirect heat exchanger (also called an evaporator). Within the coil portion of the evaporator a process fluid which is to be cooled is passed. Due to heat exchange, a portion of the liquid refrigerant is vaporized. The heavier, higher boiling point impurities tend to accumulate in the unvaporized (liquid) portion of the refrigerant within the evaporator. From the evaporator, the vaporized refrigerant is passed to a compressor, wherein the vaporous refrigerant is cornpressed; it is then passed to a cooling device wherein the compressed vapors are liquefied and recycled to the evaporator. However, due to loss of refrigerant around packing glands, valve seals, and the like. the concentration of impurities increases in the refrigerant. Therefore, more power is required in order that the compressor can return the gaseous refrigerant to a pressure sufficient to allow liquefaction of the vapors when cooled. The more power required to compress and condense the refrigerant, the more expensive the operation of the system.

Subsequent to compression and condensation of the refrigerant, the liquid refrigerant is accumulated in a suitable accumulation means. Within the accumulation means vaporous refrigerant exists in equilibrium with the liquid refrigerant above the level of the liquid refrigerant. Within this vapor phase the lighter lower boiling point impurities accumulate. In order to remove the impurities which have a lower boiling point than the refrigerant, a small stream of refrigerant vapor which contains vaporous lighter impurities is passed to an apparatus which has a bundle of condensing tubes in the upper portion, and a reservoir for accumulating refrigerant in the lower portion. The tubes are enclosed by an outer shell and liquid refrigerant is evaporated around the outside of the tubes. The vaporous refrigerant and vaporous lighter impurities mixture is passed through the tubes which results in the condensation and reliux of the refrigerant, while the lighter impurities remain in the vapor state and leave the system. The condensation of the refrigerant vapors causes the vaporization of the liquid refrigerant circulated around the tubes. This vaporized refrigerant is returned to the compressor, and the condensed liquid refrigerant accumulated in the lower portion of the apparatus is expanded which causes the refrigerant to vaporize. This expanded vaporized refrigerant is also returned to the compressor suction.

The higher boiling impurities (such as compressor lube oils) that acumulate in the liquid refrigerant within the evaporator are removed from the refrigerant by removing from the bottom of the evaporator a small stream of the liquid refrigerant which contains a high concentrationI of the higher boiling impurities. This stream is passed to a second heat exchanger wherein the refrigerant is vaporized from the liquid higher boiling impurities by heat supplied from the hot compressed refrigerant vapors which are subsequently returned to the suction of the compressor. The puried refrigerant is also returned to the compressor suction, while the liquid higher boiling point impurities are removed from the bottom of the heat exchanger. v

Although indirect or direct heat exchange may be utilized to vaporize the refrigerant from the heavier impurities, it is presently preferred to use the indirect method. The direct method presents a problem in that an increase in the quantity of refrigerant vapor associated with the heavier impurities increases the probability that the liquid heavier impurities will become entrained within the refrigerant vapor which is removed from the heat exchanger. Accordingly, the indirect method eliminates the problem of entrainment caused by vaporization within the heat exchanger, and the amount of heavier impurities in the refrigerant vapor is usually less than with the direct method.

The applicability of the present invention covers a wide range of diversified processes. Generally, any plant which requires a large refrigeration system and employs refrigerant wherein heavier and lighter impurities tend to accumulate and decrease the efficiency of cooling can use the process of the invention. Preferably, the system is used in the recovery of liquid fuels entrained in a natural gas where impure propane is the refrigerant. However, the feature of removing compressor oils and other heavier impurities by heat supplied by heat exchange with compresssed vapors of the refrigerant can be used in systems where the refrigerant is ammonia, ethylene, and the like. When propane is utilized as the refrigerant, impure makeup propane will preferably contain from 95-99 percent by weight of propane, 0.1 to percent by weight of compounds having a lower boiling point than propane, and 0.1 to 5 percent by weight of compounds having a higher boiling point than propane.

Before explaining the invention in more detail by reference to the drawing, it is pointed out that optimization of the process of the invention depends upon a number of variables. The exact temperatures and pressures employed will depend upon the extent of the cooling requirements, the particular refrigerant utilized, the concentration of the impurities in the refrigerant which are introduced into the system, the rate at which the impurities accumulate in the system, and the like. Therefore, temperature and pressure ranges, concentrations of impurities, and the exact chemical and physical properties of the refrigerants and impurities are not recited herein for the sake of brevity and understanding of the invention. Therefore, it is to be understood that the explanation of the process in reference t0 a particular refrigerant, certain impurities, tempertaure ranges, and the like should not be construed as limiting the scope of the invention. Accordingly, the drawing is explained in reference to a refrigeration system for cooling natural gas utilizing propane as the refrigerant.

Referring to the drawing, the figure is a schematic flow diagram illustrating the method of the invention. Principal apparatus are a makeup-surge tank 1, an evaporator 2, an indirect heat exchanger-separator 3, a reservoir-heat exchanger 4, separator tank 6, a compressor 7, and a cooler 8. Makeup liquid propane is passed into accumulator-surge tank 1 by conduit 11. Approximately 99 percent of the liquid propane in tank 1 is passed by conduit 12 into the outer section of evaporator 2 by line 12. The coils 29 within evaporator 2 are supplied with natural gas by line 20. Within evaporator 2, indirect heat exchange causes a portion of the liquefied propane to vaporize, and the heat exchange causes the natural gas in coils 29 to be cooled. The cooled natural gas is removed from evaporator 2 by line 10. Higher boiling impurities (heavy impurities such as lube oil, butane, pentane, etc.) accumulate in the bottom of evaporator 2 is association with liquid propane.

The vaporized propane passes from evaporator 2 through conduits 13 and 27 to separator tank 6. In separator tank 6, any liquid propane which is carried by conduit 27 settles to the bottom of separator tank 6. The liquid propane and any remaining liquid heavier impurities are periodically withdrawn from tank 6 through conduit 9. The vaporized propane passes from tank 6 through conduit 22 into compressor 7 wherein the propane is compressed. From compressor 7, the compressed gaseous propane continues through conduit 22 into cooler 8, wherein the propane is liquefied, and then passes through conduit 30 into makeup-surge tank 1.

The impurities which are present in the propane introduced into tank 1 by lines 11 and 30 include: (l) lower boiling point (than propane) compounds such as ethane, methane, and the like; (2) higher boiling point (than propane) compounds such as butanes, pentanes, and the like which are associated with propane in line 11. In addition, higher molecular weight oils which are used to lubricate compressor 7, various valves, and the like, enter tank 1 through line 30.

To remove the lighter impurities from the propane refrigerant the following steps are performed. A certain portion of the propane in tank 1 remains in the Vapor state above the liquid propane in tank 1. The lighter irnpurities, having a lower boiling point than propane, tend to accumulate within the vapor phase of the propane. As the concentration of the lighter compounds increases, a portion of the vaporized propane and the vaporized lighter compounds present in tank 1 are removed from the upper portion of tank 1 by line 28, and passed to reservoir-heat exchanger 4. The upper portion of reservoir-heat exchanger 4 contains rolled tubes 5 which are clustered, and provide a multiple condensing means for the vapor from tank 1. The tubes 5 are surrounded by a jacket 15, within which is evaporated liquid propane as a cooling fluid. The liquid propane is introduced into the jacket 15 by means of conduit 23, which transports a small amount of liquid propane from line 12 into the lower portion of the jacket 15. The vaporous mixture of propane and lighter impurities enter the reservoir-heat exchanger below the lower end of tubes 5 and pass upwardly through tubes 5. The temperature within tubes 5 is such that propane condenses upon the walls of tubes 5, and falls back into the lower portion of the reservoir-heat exchanger 4. The lighter impurities, such as methane and ethane, remain in the vapor state, and leave the upper portion of the reservoir-heat exchanger 4 through conduit 26. Due to the indirect heat exchange between the vapor in tubes 5 and the liquid propane in jacket 15, the liquid propane is vaporized and removed from the upper portion of jacket 15 by conduit 27 and is passed to separator tank 6. The liquid propane in the lower section of reservoir-heat exchanger 4 is removed therefrom, and passes through line 24 to conduit 27. Within line 24 the liquid propane is vaporized due to a pressure drop afforded by valve 37 located in line 24. Thus, the lighter impurities are removed from the refrigeration system.

Removal of the heavier impurities, such as butane and compressor oils, is accomplished -by the following steps. As previously mentioned, heavier impurities in tank 1 are carried to evaporator 2 and accumulate in the bottom thereof. As the lubricating oils and heavier hydrocarbons accumulate, the temperature within the shell section of the evaporator 2 increases, thereby decreasing the eiciency of cooling the natural gas in tubes 29. The heavy impurities associated with liquid propane are therefore withdrawn from the lower portion of evaporator 2 through conduit 14 and pass into the upper shell portion of indirect heat exchanger-separator 3. In order to conserve valuable propane which enters along with the heavy compounds, hot compressed gases are removed from line 22 and transported via conduit 1-8 into the coils 25 in heat exchanger-separator 3. By indirect heat exchange, the propane associated with the higher boiling point impurities is vaporized and removed from the upper shell portion of heat exchanger-separator 3 through conduit 16 to join conduit 27. The gases in coils 25 are returned to line 27 by conduit 19. The liquid heavy impurities are removed from the lower jacket section of heat exchanger-separator 3 by conduit 17.

As previously mentioned optimization of the system utilizing the invention depends upon a number of variables. If, for example, the concentrations of impurities in the system are relatively high, the system may be operated continuously in order to purify the refrigerant. If, however, the impurity concentrations are found to be relatively low, the refrigerant can be purified intermittently when the impurity concentrations become Sullicient to reduce the efliciency of the system. The description of the process of the invention in relation to various valves and controls will be explained in reference to an intermittent type purification operation, which, however, is not intended to limit the invention.

Makeup-surge tank 1 is provided with a temperature indicator means 57, and a pressure indicator means 56. The v,level of liquid propane within tank 1 is maintained by the addition of impure propane to tank 1 through line 11 by opening manual valve 31 located in line 11. By reading the temperature and corresponding pressure from gauges 56 and 57, the operator can determine the lconcentration of lighter impurities in the propane. When the temperature and pressure are such that the concentration of lighter impurities is, for example, 2 percent, then valve 32 located in line 28 is opened, allowing vaporized propane and the vaporized lighter impurities to pass through line 28 into reservoir-heat exchanger 4. The rate of ow through line 28 may be adjusted so that the rate of ow reduces the lighter impurities to a concentration Where the pressure and temperature in tank 1 indicates that the propane contains a concentration of impurities which does not have a deleterious effect on the refrigeration system.

Liquid level control 46, connected to the upper portion of jacket 15 and operatively connected to motor valve 26 located in line 23, controls the level of liquid propane in jacket 15. Pressure indicator-controller 44 connected to the upper portion of jacket 15 and operatively connected to motor valve 34 located in line 27, maintains the pressure in jacket 15 such that the propane remains at a constant temperature. Temperature indicator-controller 43 located in the upper portion of reservoir-heat exchanger 4 and operatively connected to motor valve 33, controls the temperature Iand therefore thepressure in reservoir-heat exchanger 4 such that vaporous propane is condensed on the walls of tubes 5 and1ighter impurities are allowed to leave the system through line 26. Liquid level control 47 connected to the lower section of reservoir-heat exchanger 4 and operatively connected to motor valve 37 maintains the level of Athe liquid propane in the lower section of reservoirheat exchanger 4 at a constant value.

Liquid level control 48, connected to evaporator 2 and operatively connected to motor valve 38 located in line 12, maintains a constant level of liquid propane inside of evaporator 2. Evaporator 2 is also provided with a temperature indicator means 58. An increase in the concentration of lubricating oils and other heavy impurities in the lower portion of evaporator 2 adversely affects the cooling etliciency of the system. The increase in concentration of heavy impurities therefore causes the temperature to rise in evaporator l2. When the temperature reaches a specilic value, manual valve 39 located in line 14 is opened, allowing the liquid heavy impurities associated with a portion of the liquid propane to ilow through line 14 and to heat exchangerseparator 3 until coils 25 are covered with the mixture which is 'indicated lby level gauge 52 connected to heat exchanger-,separator 3. Once the level of the mixture is over,coils 25, manual valve 53 located in line 18 and valve4 54 located in line 19 are opened and adjusted to provide heat through coils 25 to vaporize propane 'rin heat exchanger-separator 3. Temperature indicator 51 connected to line 16 indicates the temperature within heat exchanger-separator 3, and therefore indicates when substantially all of the propane has been vaporized. The liquid heavy impurities are then withdrawn from heat exchanger-separator 3 by conduit 17. In operation, the heavy4 impurities can be removed continuously or batchwise as conditions require.

The term batchwise refers to an operation for removing the heavy impurities through conduit 17 periodically. The operator will fill heat exchanger-Separator 3 to v the upper level as indicated on gauge 52. The liquid propane and liquid heavy impurities mixture is then subjectedx to heat exchange with the hot compressed gases` Wh`en the temperature indicator 51 indicates that substantially all of the liquid propane has been vaporized, the tank is -again lled with the mixture from evaporator 2. These steps are repeated until temperature indicator 58 attached to evaporator 2 indicates that optimum cooling is taking place within evaporator 2. When using this procedure, the removal of the liquid heavy impurities by way of conduit 17 may be postponed until the heat exchanger-separator 3 is almost full of the liquid impurities. Of course, either the batchwise or continuous process may be automated. However, an automated system has not been illustrated.

In addition, certain valves, gauges, and other apparatus'actually needed to operate the system have been intentionally omitted. Only suflicient hardware has been shown to illustrate the invention.

A more comprehensive understanding of the invention can be acquired from the following illustrative example, which, however, is not intended in any manner to limit the invention.

Example A refrigeration system substantially as depicted in the drawing was utilized to cool 35,500,000 cubic feet/day of natural gas from 125 F. in line 20 to 35 F. in line 10. The composition of the impure makeup propane in line 11 was approximately 2 percent C2, 97 percent C3, and 1 percent C4 and heavies. The liquid propane refrigerant had a pressure of 210 p.s.i.a. and a temperature of 105 F. in tank 1. When gauge 57 read 105 F., and the pressure in tank 1 was 220 p.s.i.a. as shown on gauge 56, about 3 percent of C2s were present in the vapor phase within tank 1.

In order to reduce the C2 lighter impurities, valve 32 was opened until the pressure returned to 210 p.s.i.a. in tank 1 at which time valve 32 was again closed. Approximately 1 percent of the liquid propane in line 12 was allowed to pass from line 12 into line 23, and then to jacket 15. The liquid propane in jacket 15 was maintained at a temperature of -3 F. and 36 p.s.i.a. The heat exchange in jacket 15 caused the refrigerant vapor from line 28 to be condensed and fall into the lower section of reservoir-heat exchanger 4. The liquid propane in the lower section of reservoir-heat exchanger 4 was maintained at a temperature of F. and a pressure of 200 p.s.i.a. Most of the C2 portion of vapor in line 28 was allowed to escape the system and liquefied propane was withdrawn from the reservoir-heat exchanger 4 by line 24. After passing through valve 37, the pressure was reduced to 15 p.s.i.a. and the temperature to 43 F. in line 24. The temperature and pressure of the vaporized propane in conduit 27 after passing valve 34, which resulted from the condensing of the propane vapor in tubes 5, were 43 F. and 15 p.s.i.a. Analysis of the vaporized refrigerant in line 27 prior to the compressor 7 showed that the concentration of C2 and other lighter impurities had been reduced to less than 2 percent.

The pressure and temperature in evaporator 2 of the propane were 15 p.s.i.a. and 43 F. The propane vapor from evaporator 2 in line 13 contained only trace amounts of C4s and heavies. When temperature indicator 58 read -38 F., indicating a high concentration of heavies and C4s in the lower section of evaporator 4, valve 39 in line 14 was opened. A mixture which contained about 98 percent liquid propane and 2 percent heavy impurities was passed into heat exchanger 3 until level gauge 52 indicated that the level of the liquid was above coils 2S. Valves 55 and 54 were then opened allowing hot compressed gases to flow through coils 25, vaporizing propane in heat exchanger 3. When temperature indicator 51 reached 30 F., valve 53 was closed and the heavier impurities withdrawn through line 17. Compressor 7 compressed the hot essentially pure propane vapors to 220 p.s.i.a. which increased the temperature to 165 F. Cooler 8 then liqueed the compressed gases by reducing the temperature to 105 F. prior to return of the propane to tank 1.

From the foregoing example, it can be seen that the considerable economic savings are realized using the invention. By the efficient reduction of the lighter and heavier impurities in the minimal amount of required equipment, the power requirements upon the system are maintained at a minimum. Furthermore, the invention has versatility of application in that it may be optimized for the most economical operation depending upon the extent of impurities in the refrigerant.

Various modifications of the invention will be apparent to one skilled in the art without departing from the spirit and scope thereof.

I claim:

1. A method for purifying a refrigerant of heavier impurities having a higher boiling point than said refrigerant wherein said refrigerant is alternately accumulated, vaporized, compressed, and condensed in order to cool a process uid, which comprises removing said heavier impurities by the steps of 1) passing liquid refrigerant in indirect heat exchange relation with said process fluid thereby vaporizing a portion of the liquid refrigerant;

(2) passing a portion of the liquid refrigerant of step (1) in heat exchange relation with a part of the hot compressed vaporous refrigerant from the compression step thereby producing a vaporous stream of refrigerant essentially free of said impurities;

(3) withdrawing the liquid heavier impurities from the system; and

(4) passing the vaporous heat exchanged refrigerant from steps (1) and (2) to the compression step.

2. A method according to claim 1 wherein step (2) is accomplished by indirect heat exchange of the hot compressed vaporous refrigerant and the liquid refrigerant of step (1).

3. A method -for purifying a refrigerant of heavier impurities having a higher boiler point than said refrigerant and lighter impurities having a lower boiling point than the refrgerant wherein said refrigerant is alternately accumulated, vaporized, compressed, and condensed in order to cool a process Huid, which comprises removing said heavier impurities by the steps of (1) passing liquid refrigerant in indirect heat exchange relation with said process fluid thereby vaporizing a portion of the liquid refrigerant;

(2) passing a portion of the liquid refrigerant of step 1) in heat exchange relation with hot compressed vaporous refrigerant thereby producing a vaporous stream of refrigerant essentially free of said impurities;

(3) withdrawing the liquid heavier impurities from the system;

(4) passing the vaporous heat exchanged refrigerant from steps (1) and (2) to be compressed; and removing said lighter impurities by the steps of (5) passing a portion of the accumulated refrigerant which is in a vaporized state and contains said lighter impurities in indirect heat exchange relation with liquid refrigerant at a temperature suflicient to condense said refrigerant from said lighter impurities;

(6) removing the vaporized lighter impurities from the system;

(7) expanding the condensed refrigerant from step (5) thereby vaporizing said condensed refrigerant; and

(8) passing heat exchanged refrigerant from step (5 and vaporized refrigerant from step (7) to be compressed.

4. A method according to claim 3 wherein step (2) is accomplished by passing a portion of the liquid refrigerant of step 1) which contains liquid heavier impurities in indirect heat exchange relation with hot cornpressed vaporous refrigerant.

5. A method according to claim 4 wherein said refrigerant is propane and said heavier impurities are present in said propane in the range of 0.1 to 5 percent by weight, and said lighter impurities are present in the range of .01 to 5 percent by weight.

6. A method according to claim 5 wherein said lighter impurities are methane and ethane, and said heavier impurities are butane and other higher boiling point than propane compressor oils and lubricating oils.

7. A method according to claim 6 wherein said process uid is natural gas.

References Cited UNITED STATES PATENTS 3,357,197 12/ 1967 Massengale 62-195 LLOYD L. KING, Primary Examiner U.S. Cl. X.R. 62-195, 475 

